1) I found that the source libraries checks also the version of FreeBSD, to include or not the file machine/types.h that starting from FreeBSD 5 don't exist anymore. So we have to initialize the variable "-D__FreeBSD__" to a value greater or equal to 5. I'm not sure if this is the best and most elegant way, but it works: in Makefile I added "-D__FreeBSD__=5" to the CCFLAGS definition.
2) As we already know, this application requires X11 to run. If we start double clicking the gastman icon, the application cannot display the icons (placed in the ./icons folder). If we issue the command ./gastman from the xterm, everything is okay. If somebody can explain... it's welcome.